Of course you can make a stop motion animation. But .gifs are not support by GreenfootImage. Just have a look at the API ! Supported Image typs are: JPEG, GIF and PNG.

Animated Gif is not supported by default, but this scenario http://www.greenfoot.org/scenarios/260 gives you some functionality to use animated gifs very easily :)

I think you guys are misunderstanding his question. I'm pretty sure he's asking whether your forum avatar can be animated - if so, the answer is - sadly - no.